The present invention generally relates to fuel cells. More particularly, the present invention relates to alkaline fuel cells wherein the electrodes are placed in frames to allow high air flow throughout the fuel cell while maintaining a low pressure within the fuel cell.
Fuel cells use gaseous hydrogen streams, gaseous oxygen streams, and electrolyte streams to produce power. The oxygen stream may be pure oxygen, air, or another oxygen containing mixture. Air, however, is the most abundant and attainable source of oxygen. Due to air containing only 21% oxygen, a greater air flow rate is needed when air is used as a source of oxygen, as compared to a pure oxygen stream. As a result of the high flow rate of the oxygen stream, the pressure inside the fuel cell increases. The pressure increases inside fuel cells may cause design problems for other streams entering and exiting the fuel cell because the pressure of the streams will need to be adjusted according to the pressure within the fuel cell. As such, a fuel cell allowing for a high air flow rate while maintaining a low pressure within the fuel cell is very desirable.
The present invention utilizes parallel flow of an electrolyte solution with respect to the electrodes. The electrodes are placed in frames which distribute electrolyte solution and either air or hydrogen across the electrodes. The frames containing the electrodes are sealed together and flow channels in the frames distributes air, hydrogen, and electrolyte solution across the electrodes while maintaining low pressure throughout the fuel cell. The present invention provides a compacted fuel cell design while allowing for a high air flow while maintaining a low pressure throughout the fuel cell.
As the world""s population expands and its economy increases, the atmospheric concentrations of carbon dioxide are warming the earth causing climate change. However, the global energy system is moving steadily away from the carbon-rich fuels whose combustion produces the harmful gas. Experts say atmospheric levels of carbon dioxide may be double that of the pre-industrial era by the end of the next century, but they also say the levels would be much higher except for a trend toward lower-carbon fuels that has been going on for more than 100 years. Furthermore, fossil fuels cause pollution and are a causative factor in the strategic military struggles between nations. Furthermore, fluctuating energy costs are a source of economic instability worldwide.
In the United States, it is estimated, that the trend toward lower-carbon fuels combined with greater energy efficiency has, since 1950, reduced by about half the amount of carbon spewed out for each unit of economic production. Thus, the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis of the system. It had been predicted that this evolution will produce a carbon-free energy system by the end of the 21st century. The present invention is another product which is essential to shortening that period to a matter of years. In the near term, hydrogen will be used in fuel cells for cars, trucks and industrial plants, just as it already provides power for orbiting spacecraft. But, with the problems of storage and infrastructure solved (see U.S. Pat. No. 6,305,442, entitled xe2x80x9cA Hydrogen-based Ecosystemxe2x80x9d filed on Nov. 22, 1999 for Ovshinsky, et al., which is herein incorporated by reference and U.S. Pat. No. 6,193,929, entitled xe2x80x9cHigh Storage Capacity Alloys Enabling a Hydrogen-based Ecosystemxe2x80x9d, filed on Nov. 6, 1999 for Ovshinsky et al., which is herein incorporated by reference), hydrogen will also provide a general carbon-free fuel to cover all fuel needs.
A dramatic shift has now occurred, in which the problems of global warming and climate change are now acknowledged and efforts are being made to solve them. Therefore, it is very encouraging that some of the world""s biggest petroleum companies now state that they want to help solve these problems. A number of American utilities vow to find ways to reduce the harm done to the atmosphere by their power plants. DuPont, the world""s biggest chemicals firm, even declared that it would voluntarily reduce its emissions of greenhouse gases to 35% of their level in 1990 within a decade. The automotive industry, which is a substantial contributor to emissions of greenhouse gases and other pollutants (despite its vehicular specific reductions in emissions), has now realized that change is necessary as evidenced by their electric and hybrid vehicles.
Hydrogen is the xe2x80x9cultimate fuel.xe2x80x9d In fact, it is considered to be xe2x80x9cTHExe2x80x9d fuel for the future. Hydrogen is the most plentiful element in the universe (over 95%). Hydrogen can provide an inexhaustible, clean source of energy for our planet which can be produced by various processes. Utilizing the inventions of subject assignee, the hydrogen can be stored and transported in solid state form in trucks, trains, boats, barges, etc. (see the ""810 and ""497 applications).
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Researchers have been actively studying fuel cells to utilize the fuel cell""s potential high energy-generation efficiency. The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Presently most of the fuel cell R and D focus is on P.E.M. (Proton Exchange Membrane) fuel cells. The P.E.M. fuel cell suffers from relatively low conversion efficiency and has many other disadvantages. For instance, the electrolyte for the system is acidic. Thus, noble metal catalysts are the only useful active materials for the electrodes of the system. Unfortunately, not only are the noble metals costly, they are also susceptible to poisoning by many gases, and specifically carbon monoxide (CO). Also, because of the acidic nature of the P.E.M fuel cell, the remainder of the materials of construction of the fuel cell need to be compatible with such an environment, which again adds to the cost thereof. The proton exchange membrane itself is quite expensive, and because of its low conductivity, inherently limits the power performance and operational temperature range of the P.E.M. fuel cell (the PEM is nearly non-functional at low temperatures, unlike the fuel cell of the instant invention). Also, the membrane is sensitive to high temperatures, and begins to soften at 120xc2x0 C. The membrane""s conductivity depends on water and dries out at higher temperatures, thus causing cell failure. Therefore, there are many disadvantages to the P.E.M. fuel cell which make it somewhat undesirable for commercial/consumer use.
The conventional alkaline fuel cell has some advantages over P.E.M. fuel cells in that they have higher operating efficiencies, they use less expensive materials of construction, and they have no need for expensive membranes. The alkaline fuel cell also has relatively higher ionic conductivity in the electrolyte, therefore it has a much higher power capability. Unfortunately, conventional alkaline fuel cells still suffer from certain disadvantages. For instance, conventional alkaline fuel cells still use expensive noble metals catalysts in both electrodes, which, as in the P.E.M. fuel cell, are susceptible to gaseous contaminant poisoning. While the conventional alkaline fuel cell is less sensitive to temperature than the PEM fuel cell, the active materials of conventional alkaline fuel cell electrodes become very inefficient at low temperatures.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous hydrogen electrode and oxygen electrode and brought into surface contact with the electrolytic solution. The particular materials utilized for the hydrogen electrode and oxygen electrode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the hydrogen electrode occurs between the hydrogen fuel and hydroxyl ions (OHxe2x88x92) present in the electrolyte, which react to form water and release electrons:
H2+2OHxe2x88x92xe2x86x922H2O+2exe2x88x92. 
At the oxygen electrode, the oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OHxe2x88x92):
O2+2H2O+4exe2x88x92xe2x86x924OHxe2x88x92. 
The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
The catalyst in the hydrogen electrode of the alkaline fuel cell has to not only split molecular hydrogen to atomic hydrogen, but also oxidize the atomic hydrogen to release electrons. The overall reaction can be seen as (where M is the catalyst):
M+H2xe2x86x922 MHxe2x86x92M+2H++2exe2x88x92. 
Thus the hydrogen electrode catalyst must efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional hydrogen electrode material, the dissociated hydrogen atoms are transitional and the hydrogen atoms can easily recombine to form molecular hydrogen if they are not used very quickly in the oxidation reaction. With the hydrogen storage electrode materials of the inventive instant startup fuel cells, the atomic hydrogen is immediately captured and stored in hydride form, and then used as needed to provide power.
In addition to being catalytically efficient on both interfaces, the catalytic material must be resistant to corrosion in the alkaline electrolyte environment. Without such corrosion resistance, the electrodes would quickly lose efficiency and the cell will die.
One prior art fuel cell hydrogen electrode catalyst is platinum. Platinum, despite its good catalytic properties, is not very suitable for wide scale commercial use as a catalyst for fuel cell hydrogen electrodes, because of its very high cost. Also, noble metal catalysts like platinum cannot withstand contamination by impurities normally contained in the hydrogen fuel stream. These impurities can include carbon monoxide which may be present in hydrogen fuel.
The above contaminants can cause what is commonly referred to as a xe2x80x9cpoisoningxe2x80x9d effect. Poisoning occurs where the catalytically active sites of the material become inactivated by poisonous species invariably contained in the fuel cell. Once the catalytically active sites are inactivated, they are no longer available for acting as catalysts for efficient hydrogen oxidation reaction at the hydrogen electrode. The catalytic sites of the hydrogen electrode therefore are reduced since the overall number of available catalytically active sites is significantly lowered by poisoning. In addition, the decrease in catalytic activity results in increased over-voltage at the hydrogen electrode and hence the cell is much less efficient adding significantly to the operating costs. Over-voltage is the difference between the actual working electrode potential and it""s equilibrium value, the physical meaning of over-voltage is the voltage required to overcome the resistance to the passage of current at the surface of the hydrogen electrode (charge transfer resistance). The over-voltage represents an undesirable energy loss which adds to the operating costs of the fuel cell.
In related work, U.S. Pat. No. 4,623,597 (xe2x80x9cthe ""597 patentxe2x80x9d) and others in it""s lineage, the disclosure of which is hereby incorporated by reference, one of the present inventors, Stanford R. Ovshinsky, described disordered multi-component hydrogen storage materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor made (i.e., atomically engineered) to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are amorphous, microcrystalline, intermediate range order, and/or polycrystalline (lacking long range compositional order) wherein the polycrystalline material includes topological, compositional, translational, and positional modification and disorder. The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites.
The disordered electrode materials of the ""597 patent were formed from lightweight, low cost elements by any number of techniques, which assured formation of primarily non-equilibrium metastable phases resulting in the high energy and power densities and low cost. The resulting low cost, high energy density disordered material allowed the batteries to be utilized most advantageously as secondary batteries, but also as primary batteries.
Tailoring of the local structural and chemical order of the materials of the ""597 patent was of great importance to achieve the desired characteristics. The improved characteristics of the hydrogen electrodes of the ""597 patent were accomplished by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix to create a desired disordered material. Disorder permits degrees of freedom, both of type and of number, within a material, which are unavailable in conventional materials. These degrees of freedom dramatically change a materials physical, structural, chemical and electronic environment. The disordered material of the ""597 patent have desired electronic configurations which result in a large number of active sites. The nature and number of storage sites were designed independently from the catalytically active sites.
Multiorbital modifiers, for example transition elements, provided a greatly increased number of storage sites due to various bonding configurations available, thus resulting in an increase in energy density. The technique of modification especially provides non-equilibrium materials having varying degrees of disorder provided unique bonding configurations, orbital overlap and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural rearrangement occurs during charge/discharge cycles or rest periods there between resulting in long cycle and shelf life.
The improved battery of the ""597 patent included electrode materials having tailor-made local chemical environments which were designed to yield high electrochemical charging and discharging efficiency and high electrical charge output. The manipulation of the local chemical environment of the materials was made possible by utilization of a host matrix which could, in accordance with the ""597 patent, be chemically modified with other elements to create a greatly increased density of electro-catalytically active sites and hydrogen storage sites.
The disordered materials of the ""597 patent were designed to have unusual electronic configurations, which resulted from the varying 3-dimensional interactions of constituent atoms and their various orbitals. The disorder came from compositional, positional and translational relationships of atoms. Selected elements were utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
The internal topology that was generated by these configurations also allowed for selective diffusion of atoms and ions. The invention that was described in the ""597 patent made these materials ideal for the specified use since one could independently control the type and number of catalytically active and storage sites. All of the aforementioned properties made not only an important quantitative difference, but qualitatively changed the materials so that unique new materials ensued.
Disorder can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions of the material. The disorder also can be introduced by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, or by introducing regions of an amorphous phase or phases in addition to regions of a crystalline phase or phases. The interfaces between these various phases can provide surfaces which are rich in local chemical environments which provide numerous desirable sites for electrochemical hydrogen storage.
These same principles can be applied within a single structural phase. For example, compositional disorder is introduced into the material which can radically alter the material in a planned manner to achieve improved and unique results, using the Ovshinsky principles of disorder on an atomic or microscopic scale.
The present invention discloses an improved fuel cell. The fuel cell of the present invention allows for a high air flow rate, when air is used as the source of oxygen, while maintaining low pressure throughout the fuel cell. The cell design also provides mechanical support within the fuel cell. The fuel cell contains at least one hydrogen electrode in contact with a hydrogen stream, at least one oxygen electrode in contact with an oxygen stream, and at least one electrolyte chamber in contact with the hydrogen electrode and the oxygen electrode. The hydrogen stream may be composed of gaseous hydrogen and the oxygen stream may be composed of pure oxygen or air from the environment. An electrolyte solution, such as potassium hydroxide, flows through the electrolyte chambers and contacts the hydrogen electrodes and the oxygen electrode. The fuel cell also contains multiple rubber compression plate used to distribute oxygen and hydrogen to the electrodes and help maintain mechanical support in the fuel cell while allowing for expansion and contraction of the hydrogen electrodes.
The hydrogen and the oxygen electrodes are each placed in a frame. The electrolyte chamber is placed between the hydrogen electrode and the oxygen electrode and the frames are adhered together forming an electrode chamber. The frames provide flow channels allowing the electrolyte solution to flow between the hydrogen and oxygen electrode. The frames are configured to uniformly distribute the electrolyte solution between the electrodes.
Compression are placed outside the frames within the fuel cell. The compression plates are configured to uniformly distribute hydrogen or oxygen to the electrodes. The compression plates have a series of flow channels which evenly distribute the hydrogen and oxygen across the respective electrodes. The compression plates are also adapted to absorb expansion of the hydrogen electrode while providing mechanical support within the fuel cell. The compression plate may be comprised of rubber or another elastomeric compound capable of absorbing the expansion of the hydrogen electrodes.
The electrolyte chambers may be composed of a porous support structure disposed between a pair of membranes. The membranes prevent excess electrolyte solution from contacting the hydrogen electrodes and the oxygen electrode. The membranes also prevent the oxygen stream and the hydrogen stream from penetrating into the electrolyte. The porous support structure may be an expanded polymer sheet. The polymer may be of polyolefin or another rigid polymer. The electrolyte chambers contact an electrolyte contacting surface of the hydrogen electrodes and the oxygen electrodes. The electrolyte chamber is adapted to provide mechanical support within the fuel cell and provide a pathway for uninterrupted flow of the electrolyte solution throughout the fuel cell. The electrolyte chambers allow the electrolyte solution to contact the hydrogen electrodes and the oxygen electrodes.
The hydrogen electrode may be composed of an anode active material having hydrogen storage capacity. The hydrogen electrode has a hydrogen contacting surface, an electrolyte solution contacting surface, and bulk of an active anode material. The bulk of said anode active material is disposed between the hydrogen contacting surface and the electrolyte contacting surface. The hydrogen contacting surface is adapted to dissociate and absorb gaseous hydrogen. The bulk of said anode active material is adapted to store said absorbed hydrogen. The electrolyte contacting surface is adapted to react said stored hydrogen with an electrolyte solution.
The hydrogen electrode may comprise an anode active material layer, a porous polytetrafluoroethylene (PTFE) layer, and a current collector grid. The anode active material layer may be composed of a mixture of AB5 type of alloy, AB2 type of alloy, Raney nickel, graphite, and PTFE powder. The anode active material layer is disposed between the current collector grid and the polytetrafluoroethylene layer. The anode active material layer may be dispersed throughout the current collector grid. Examples of current collector grids include, but are not limited to, mesh, grid, matte, expanded metal, foil, foam and plate. To reduce the ohmic drop and better distribute current, the mesh may have 40 wires per inch running horizontally and 20 wires per inch running vertically. The wires comprising the mesh may have a diameter between 0.005 inches and 0.01 inches, preferably between 0.005 inches and 0.008 inches. The current collector grid may be composed of a conductive metal such as nickel.
The oxygen electrode has an oxygen contacting surface, an electrolyte solution contacting surface, and a bulk of a cathode active material. The bulk of the cathode active material is disposed between the oxygen contacting surface and the electrolyte contacting surface. The oxygen contacting surface is adapted to dissociate and absorb gaseous oxygen. The bulk of said cathode active material is adapted to store the absorbed oxygen. The electrolyte contacting surface is adapted to react the stored oxygen with an electrolyte solution.
The oxygen electrode is composed of a gas diffusion layer, a catalyst layer, a polytetrafluoroethylene layer, and a current collector grid. The catalyst layer is disposed between the gas diffusion layer and the current collector grid. The gas diffusion layer is disposed between the catalyst layer and the polytetrafluoroethylene layer. The polytetrafluoroethylene layer is in intimate contact with the oxygen stream. The current collector grid is in intimate contact with said electrolyte stream. The current collector grid may be a mesh, grid, matte, expanded metal, foil, foam and plate. To reduce the ohmic drop and better distribute current, the mesh may have 40 wires per inch running horizontally and 20 wires per inch running vertically. The wires comprising the mesh may have a diameter between 0.005 inches and 0.01 inches, preferably between 0.005 inches and 0.008 inches. The current collector may be composed of a conductive material such as nickel. The catalyst layer may be dispersed throughout the current collector grid. The gas diffusion layer may be composed of a mixture of polytetrafluoroethylene and carbon black. The catalyst layer may be composed of a mixture of a mixture of polytetrafluoroethylene and carbon black, additional carbon black, graphite, silver oxide, or other catalysts. The silver oxide may contain a lithium aluminum alloy, gallium, molybdenum, or nickel.